(1) Field of the Invention
The present invention relates to an image forming apparatus having a rotary developing apparatus which rotatably accommodates a plurality of developer units, and a control method of the image forming apparatus.
(2) Description of the Related Art
One type of image forming apparatus includes a rotary developing apparatus that accommodates developer units for respective toner colors of Cyan (C), Magenta (M), Yellow (Y), and Black (K) in a rotatable rack (hereinafter referred to as “rotary rack”), and that drives the rotary rack to rotate in order to bring a targeted one of the developer units to a developing position. The reduction in size and manufacturing cost has been demanded for image forming apparatuses provided with such rotary developing apparatuses. Accordingly, a simple and compact structure is demanded for the rotary developing apparatuses.
Usually, each developer unit has a toner storage and a developing chamber that receives toner from the toner storage and supplies the toner to a developing roller. However, for the purpose of the compact and simple structure mentioned as above, a toner supply mechanism such as a stirring blade used for delivering the toner from the toner storage to the developing chamber tend to be omitted.
The simplified rotary developing apparatus works as follows. When one of the developer units is located within a predetermined rotation range while the rotary rack being rotated, the toner in the toner storage naturally falls into the developing chamber through an opening of a partition part (partition wall) between the toner storage and the developing chamber. Thus, the toner is supplied from the toner storage to the developing chamber (For an example, see Japanese Laid-Open Patent Application No. 2006-126554).
When an image forming apparatus including such a rotary developing apparatus forms a full-color image, the rotary rack rotates by a predetermined angle (90 degrees) at a time to develop an image of a page in each color, thereby performing the multi-layer transfer. Accordingly, if each developing chamber contains at least the amount of toner sufficient to develop an image of one page, it is not necessary to provide a forced supply of toner during image formation. This is because the rotary rack always makes a 360° rotation before forming a color image of the next page, and during the 360° rotation, within a particular rotation range, the toner is supplied from the toner storage to the developing chamber by falling.
However, when numerous monochrome image forming jobs in black are executed consecutively, the rotary rack remains in the same position without rotating, and this may cause the developing chamber to become empty of the black toner. One way to avoid this is to suspend the execution of the monochrome image forming job, lead the toner to naturally fall into the developing chamber by rotating the rotary rack for 360° at a predetermined rotational speed, and subsequently resume the monochrome image forming job.
However, only a limited amount of toner falls into the developing chamber from the toner storage during a simple 360° rotation of the rotary rack. Besides, the amount of toner which falls into the developing chamber during a 360° rotation decreases with decrease of the amount of toner in the toner storage. As a result, stable toner supply to the developing chamber is impossible.
If the amount of toner supplied to the developing chamber is small, even if the monochrome image forming job is resumed, the developing chamber soon reaches the toner-empty state which requires the job to be suspended again and the rotary rack to make another 360° roll. This results in a lower efficiency in processing monochrome image forming jobs. Moreover, in a case where the toner-empty state is judged on the premise that a steady amount of toner is supplied to the developing chamber, the developing chamber reaches the toner-empty state earlier than an estimated time after the monochrome image forming is resumed, which is likely to cause defective printing such as scraped images.